Hierarchical Environmental Exposure Transforms Zeolitic Imidazolate Framework-8 and Increases Toxicity in Daphnia...

Metal-organic frameworks (MOFs) are increasingly deployed in environmental technologies, yet their fate and hazard under realistic multistep exposure scenarios remain poorly constrained. Here, we track hierarchical transformations of nanoscale ZIF-8 (Zeolitic Imidazolate Framework-8) across an exposure cascade spanning atmospheric aging (air and reactive gases O₃/NO₂), aqueous aging in environmentally and biologically relevant media, and ingestion by the freshwater crustacean Daphnia magna. Synchrotron Zn K-edge X-ray absorption spectroscopy (XAS), micro-X-ray fluorescence (μ-XRF), X-ray photoelectron spectroscopy (XPS), and electron microscopy show that gas-phase exposure produces only minor surface perturbations, whereas aqueous contact drives pronounced medium-dependent restructuring, including nitrogen depletion and oxygen enrichment at the surface and time-resolved dissolved Zn release with chemistry-imposed plateaus. In vivo, Zn speciation diverges from the pristine Zn-N fingerprint; an unexposed endogenous Zn baseline and linear combination fitting (LCF) indicate a mixture of endogenous Zn with transformed Zn pools dominated by O/P/S-type coordination environments. Acute ecotoxicity assay demonstrates strong concentration dependence (48 h immobilization EC₅₀ ≈0.5 μg mL^-1), and chronic exposure at 0.10 μg mL^-1 reduces cumulative brood production with increased adult mortality over 24 days. Mechanistically, fractionated toxicity assays show that washed aged particles/precipitates and whole aged suspensions are more potent than particle-free filtrates, indicating that particle-associated transformed Zn pools contribute substantially beyond dissolved Zn alone. Together, these results show that ZIF-8 risk emerges from its sequential transformation trajectory rather than its pristine state, motivating tiered aging protocols coupled to in vivo speciation and fractionated hazard testing for MOF safety assessment.